The present invention relates to a sealing material for each cell of a fuel cell assembly, which comprises a liquid resin composition capable of forming a low-gas-permeable and elastic sealing layer at a bonded surface among separators, a pair of electrodes and an ion exchange resin serving as a solid electrolyte by three-dimensional crosslinking, thereby airtightly sealing them completely.
A fuel cell is an apparatus for directly converting an energy of a fuel into an electric energy. For example, an electromotive force can be obtained by the electrochemical reaction at both electrodes with supplying a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas to an anode and a cathode, respectively. This electrochemical reaction can be expressed by the reaction of Equation (1) at the anode, the reaction of Equation (2) at the cathode and the reaction of Equation (3) in the whole cell.
xe2x80x83H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
(xc2xd)O2+2H+2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+(xc2xd)O2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
A fuel cell generally comprises a pair of electrodes and a solid electrolyte membrane disposed therebetween. A hydrogen-containing fuel gas is supplied to the anode electrode, while an oxygen-containing oxidizing gas is supplied to the cathode electrode separately and isolatedly from the hydrogen-containing fuel gas. If they are not separated sufficiently and happen to mix each other, an electricity generating efficiency lowers inevitably.
A fuel cell is generally a fuel cell assembly having unit cells, each having a pair of electrodes as a principal unit, stacked one after another. Each unit cell has a pair of electrodes and a solid electrolyte membrane sandwiched therebetween and, moreover, has this sandwiched structure disposed between gas impermeable separators.
These separators serve to prevent mixing of gases between two adjacent cells. The solid electrolyte membrane acts a role of separating a fuel gas and a:n oxidizing gas to be fed into each of the unit cells.
As a conventional airtight sealing method, a technique of disposing a groove at the end of the separator and disposing an O-ring at this groove, thereby preventing these gases, which are to be supplied to opposite sides of a solid electrolyte membrane, from being mixed is disclosed in JP-A-6-119930 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cUnexamined published Japanese patent application) and JP-A-6-68884.
As a solid electrolyte, an ion exchange resin membrane is used for a small-sized fuel cell. Since the ion exchange resin membrane exhibits electrically conductive behavior when it is wet, the ion exchange resin membrane is kept wet by supplying moisture to each cell of the fuel cell during operation. In other words, the ion exchange resin membrane must have two functions, that is, a function for separating a fuel gas from an oxidizing gas and a function for maintaining a wet state. An ion exchange resin membrane made of a fluorine resin can be mentioned as a preferable ion exchange resin membrane equipped with these functions.
An airtight sealing technique with an adhesive instead of the above-described O-ring is disclosed in JP-A-7-249417, while a thermocompression bonding technique of an ion exchange resin membrane is disclosed in JP-A-6-119928. However, the ion exchange resin membrane made of a fluorine resin generally has poor adhesion, so that assured airtight sealing cannot be attained by the above-described techniques.
As for the case where an epoxy resin adhesive is used for bonding, a technique of improving the adhesion of an ion exchange resin membrane by subjecting its bonded surface to ion exchange pretreatment is disclosed in JP-A-9-199145. This pretreatment improves adhesion, but lowers electric conductivity, leading to a reduction in electromotive force of a fuel cell.
With regard to a sealing material composition which undergoes addition polymerization through hydrosilylation, a perfluoropolyether-based composition and a polyisobutylene-based composition are disclosed in JP-A-8-269317 and JP-A-6-279691, respectively.
If each constituting element of a fuel cell has a reduced film thickness and each unit cell becomes thin, the number of unit cells to be stacked in a predetermined space can be enlarged, resulting in an increased output of the fuel cell. However, airtight sealing of a separator and an ion exchange resin membrane with an O-ring requires an extra thickness for disposing a groove for the O-ring, which disturbs the thinning and increased output of the fuel cell. Moreover, since airtight sealing effects are not exhibited unless the ion exchange resin membrane is compressed by a clamping force by the O-ring, the size of the membrane must necessarily be made larger than the groove for the O-ring.
It is impossible to clamp the vicinity of the electrode by the O-ring, because the electrodes, which are made of a porous material to permit diffusion of gases in the electrodes and hence are remarkably fragile, are broken if strongly clamped by the O-ring.
Consequently, the airtight sealing method by the O-ring requires an increase in the area of the ion exchange resin membrane, thereby disturbing the size reduction of a fuel cell. In addition, this method requires a high cost for the production of a fuel cell assembly owing to costly cutting work of the O-ring groove on the separator in addition to a markedly expensive ion exchange resin membrane.
Concerning the airtight sealing with an epoxy resin adhesive, it requires ion exchange pretreatment-of the bonded surface of the ion exchange resin membrane for improving adhesion. In addition, this method is accompanied with a problem of an elution of impurity ions such as chlorine ion from the epoxy resin. If the membrane is contaminated with impurity ions eluted from the epoxy resin, the electric conductivity of the membrane lowers, leading to deterioration in the electromotive force of each unit cell. As a result, the total electromotive force of the fuel cell assembly having unit cells stacked in series is reduced.
The thermocompression bonding of an ion exchange resin membrane also requires ion exchange pretreatment of the membrane and therefore is not free from the above-described problems. The thermocompression bonding tends to damage the ion exchange resin membrane and the thus damaged membrane presumably may be short-circuited owing to a difference in the internal pressure upon operation of a fuel cell.
The air tightness of a fuel cell sealed by an O-ring or epoxy resin adhesive is incomplete because of the above-described reasons, so that when it is used with being mounted on an automobile, etc., gas leakage tends to occur owing to the vibration upon traveling.
The sealing material according to the present invention realizes a minimization in the area of the ion exchange resin membrane of each unit cell and also a reduction in its film thickness, which makes it possible to decrease the size of a fuel cell and prevent reduction in the electromotive power. In addition, use of the sealing material of the present invention permits formation of an elastic sealing layer on the bonded surfaces among the ion exchange resin membrane, separators and a pair of electrodes, whereby highly reliable air tightness can be attained and the wet state of the membrane can be kept completely.
When the sealing material of the present invention is employed upon production of a fuel cell using an ion exchange resin membrane, ion exchange pretreatment for improving the adhesion to the membrane or the use of another sealing member such as O-ring becomes unnecessary, and in addition, the membrane is free from the problem of contamination with impurity ions eluted from an epoxy resin adhesive.
In the present invention, therefore, an ion exchange resin membrane can be completely adhered and sealed airtightly with separators or a pair of electrodes as compared to the conventional technique; a size reduction and thickness decrease of a fuel cell assembly can be attained as compared to the system using an O-ring; and the working step can be shortened and cost can be reduced as compared to the system using an epoxy resin adhesive, because the adhesion improving pretreatment is not necessary.
The sealing material according to the present invention has features, for example, a) a markedly low-gas-permeability, b) excellent tightness/adhesion with an ion exchange resin membrane, c) less elution of impurity ions after hardening, and d) a low moisture permeability. Therefore, it is possible to airtightly seal the ion exchange resin membrane completely with separators or a pair of electrodes without causing a deterioration in the performances of the membrane.
In the present invention, it is preferred that each of the separators or a pair of electrodes to be bonded with an ion exchange resin membrane has a roughened surface, because this increases the adhesion area of the sealing material in unevenness of the roughened surface, which enables stronger adhesion.
The sealing material according to the present invention is a low-gas-permeable and reactive liquid resin composition. The sealing material is three-dimensionally crosslinked after applied to bonded surfaces among the members of each unit cell, i.e., separators, a pair of electrodes and an ion exchange resin membrane serving as a solid electrolyte, thereby airtightly sealing them. The sealing material comprises an addition-polymerizable oligomer which has, as the backbone in the molecule, either a linear polyisobutylene or perfluoropolyether structure and has an alkenyl group at least at each end, B) a hardener containing, in its molecule thereof, at least two hydrogen atoms each bonded to a silicon atom, and C) a hydrosilylation catalyst.
One of the two kinds of the addition polymerizable oligomers as the component A) has, as the backbone in its molecule, a linear polyisobutylene structure and has, at least at each end, a reactive group. This addition polymerizable oligomer preferably has a molecular weight of 500 to 100000 and a total amount of isobutylene-derived recurring units of not lower than 50 wt. %. The addition polymerizable oligomer may be formed entirely of isobutylene units, or may be a copolymer with 50 wt. % or less, per molecule, of a polyolefin such as polyethylene and polypropylene or a polydiene such as polybutadiene and polyisoprene.
The other one kind of the addition polymerizable oligomers as the component A) has, as the backbone in its molecule, a linear perfluoropolyether structure and has, at least at each end, a reactive group. This addition polymerizable oligomer has 3 to 400 recurring units shown below.
CF2Oxe2x80x94, CF2CF2Oxe2x80x94, CF2CF2CF2Oxe2x80x94, CF(CF3)CF2Oxe2x80x94, CF2CF2CF2CF2Oxe2x80x94, C(CF3)2Oxe2x80x94
Such an addition polymerizable oligomer having a perfluoropolyether structure has a viscosity ranging from 25 to 1,000,000 mm2/s.
Although there is no particular limitation imposed on the hardener as the component B), insofar as it contains, in its molecule thereof, at least 2 hydrogen atoms each bonded to a silicon atom, a hardener which has, by itself, low gas permeability and is compatible with the addition polymerizable oligomer is preferred. In other words, when the backbone of the addition polymerizable oligomer has a polyisobutylene structure, the hardener is preferred to have a backbone of a polyisobutylene structure, while when the backbone of the addition polymerizable oligomer has a perfluoropolyether structure, the hardener is preferred to have a backbone of a perfluoropolyether structure. Such a combination is most preferred.
It is preferred that the amount of the hydrosilyl group in the hardener falls within a range of 0.5 to 5 moles per 1 mole of the alkenyl group of the addition polymerizable oligomer. The hardener preferably has a molecular weight ranging from 100 to 30000.
As the component C), ordinarily employed hydrosilylation catalysts such as chloride of platinum, titanium, palladium or rhodium may be used. Specific examples of the preferred catalyst include platinum chloride, platinum-vinyl siloxane complex, platinum-phosphine complex, platinum-phosphite complex, platinum-alcoholate complex and platinum-olefin complex.
To the sealing material of the present invention, a known material such as filler, extender pigment, antioxidant or surfactant may be added as needed within an extent not causing a problem of elution of impurity ions.
The sealing material of the present invention is used by applying it in the liquid form to the bonding surfaces of each of separators, a pair of electrodes and an ion exchange resin serving as a solid electrolyte, assembling them into a unit cell, and three-dimensionally crosslinking the sealing material under heating or by allowing it to stand at the normal temperature, to thereby form an elastic sealing layer from the sealing material on the bonded surfaces. A plurality of the unit cells thus fabricated are stacked one after another by applying a compressive force that is larger than the clamping force for fixing in the above-described crosslinking step. Stacking while under compression makes it possible to enhance the air tightness of the stacked structure, because the sealing material crosslinked by addition polymerization undergoes cure shrinkage to some extent.